Several methods for measurement of static and time varying magnetic fields are known, including positioning at least one piezoelectric (PZT) layer and at least one magnetostrictive (MNS) layer, contiguous to each other, and measurement of a voltage signal induced in a PZT layer in response to impression of a magnetic field on an MNS layer. Mermelstein, in U.S. Pat. Nos. 4,769,599 and 5,130,654, and Podney, in U.S. Pat. No. 5,675,252, discuss several such methods. Li and O""Handley, in xe2x80x9cAn Innovative Passive Solid-State Magnetic Sensorxe2x80x9d, Sensors, October 2000, pp. 52-54, compare performance of a Faraday effect sensor, a Hall effect sensor and other magnetic field sensors with performance of a passive solid state magnetic field sensor that uses a combined magnetostrictive layer and piezoelectric layer.
When these methods are applied to attempt to measure the value of an unknown magnetic field, the variation of induced voltage signal is often found to be relatively insensitive to the value of the magnetic field that is present. What is needed is a new approach that provides a measured value that is more sensitive to the value Hu=|Hu| of an unknown magnetic field that is present at the sensor. Preferably, the measured value should have a unique value for each value Hu of the magnetic field that is present.
These needs are met by the invention, which provides several embodiments of sensors for static and for dynamic (time varying) magnetic fields. In a first embodiment, one or more layers of a magnetostrictive (MNS) material with selected orientation is mechanically in contact with one or more layers of a piezoresistive (PZR) material, and a substantially static electrical current flows through the PZR material. Impression of a magnetic field Hu on the MNS layer(s) causes the MNS layer(s) to change, or to attempt to change, a dimension in at least one selected direction (e.g., perpendicular to or parallel to an MNS layer-PZR layer interface) and produces a strain or a stress across an MNS layer-PZR layer interface. This change in strain at the interface causes a change in the resistance to flow of electrical charge within the PZR layer, and this change is resistance is measured by a voltmeter or similar instrument, when a constant current is provided within the PZR layer.
In a second embodiment, one or more layers of a magnetostrictive (MNS) material with selected orientation is mechanically in contact across an interface with one or more layers of a piezoelectric (PZT) material, and a time varying electrical current flows in a coil that surrounds the MNS-PZT structure. The time varying electrical current induces a small, time varying, known magnetic field xcex94H(t) on the MNS layer, in combination with a magnetic field with unknown value Hu impressed on the MNS layer. A time varying combined voltage value Vu+xcex94V(t) is developed in the PZT layer, including a contribution Vu from the unknown magnetic field and a contribution xcex94V(t) from the known magnetic field. The combined voltage value Vu+xcex94V(t) is sampled at two or more selected times. An averaged value of the voltage signal, estimated by 0.5{Vu+xcex94V(t)}max+0.5{Vu+xcex94V(t)}min, is proportional to the unknown magnetic field value Hu for the field that is present. In both the first and second embodiments, the unknown magnetic field Hu may be static or may change with time at a rate up to about 1 MHz.